This invention relates to position measuring instruments, and more particularly relates to digital compasses and the like specifically adaptable for use in measuring angular displacement relative to the earth's magnetic field.
Various instruments have been devised for determining compass directions and converting same into a numerical display of that reading. Such instruments conventionally include a pointer and graduated dial associated with a floating magnet so as to permit measurement of angular displacement for a given sighting relative to the earth's magnetic field. Digit code discs have also been utilized to follow movement of floating magnets and together with a sensor unit and associated logical circuitry provide a digital readout of the compass direction. For instance, this has been done by a combination of a light source and light sensitive detectors to sense incremental displacement of the digital code strip away from a reference line and counting the increments of movement away from the reference line through a counting circuit, for instance, as disclosed in U.S. Pat. No. 3,746,842 to Fowler. Other coded disc position indicating instruments are disclosed in United States Letters Patent No. 3,772,503 and 3,833,901 to Fowler. In addition, conversion of the incremental movement through the counting circuit into a digital display has also been suggested in U.S. Pat. Nos. 3,825,823 to Rovner and 3,746,843 to Wesner.
Typically, digital compasses are devised for determining the reference point of the compass; or in other words the horizontal angle between the direction of the pointer and magnetic north has been through absolute encoding systems which require that the digit code disc which is rotated by interaction of the attached magnets on the disc and the earth's magnetic field have a plurality of tracks or channels which would customarily include as many as twelve tracks for three-digit BCD information plus one strobe track. The advantage of this arrangement was that the position information was available immediately upon energization of the circuit since the disc carried the absolute angle information. However, this approach required a disc diameter which was determined by the minimum mounting distance of available sensors and emitters and the number of channels of information therefore increasing greatly the minimum permissible size of the compass. See for example United States Letters Patent No. 3,950,859, issued to Melvin G. Kramer, the applicant of the present invention and assigned to the assignee of this application.
It is therefore highly desirable to provide an incremental encoding system for a digital compass which will permit greatly reduced size of the digit code disc and sensor units and as a result greatly reduce the size, weight and power requirements of the entire compass unit. It is further highly desirable to provide such a system which is capable of immediately sensing the zero or reference position when the power is turned on and for accurately counting the direction and degree of displacement of the disc from that zero or reference point to provide an immediate reading of the compass direction.